According to the prior art, automatic transmissions, particularly for motor vehicles, comprise planetary gearsets that are shifted by way of frictional, and/or gearshifting elements, such as clutches and brakes, and usually connected to a starting element, like a hydrodynamic torque converter or a fluid clutch, subject to slip action and optionally provided with a lockup clutch.
Within the scope of the Applicant's DE 101 15 983 A1, for example, a multi-speed transmission is described with one input shaft connected to a front-mounted set, with an output shaft connected to a rear-mounted set and with a maximum of seven gearshifting elements, the selective shifting of which accomplishes that at least seven forward gears can be shifted without range shifts The front-mounted set consists of a shiftable or non-shiftable planetary gearset or of a maximum of two non-shiftable planetary gearsets connected to one another. The rear-mounted set is configured as a two-carrier, four-shaft transmission with two shiftable planetary gearsets featuring four free shafts. The first free shaft of this two-carrier, four-shaft transmission is connected to the first gearshifting element; the second free shaft to the second and third gearshifting element; the third free shaft to the fourth and fifth gearshifting element, and the fourth free shaft to the output shaft. For a multi-speed transmission with a total of six gearshifting elements, according to the present invention, it is proposed that the third free shaft or the first free shaft of the rear-mounted set be additionally connected to a sixth gearshifting element. For a multi-speed transmission with a total of seven gearshifting elements, according to the present invention, it is proposed that the third free shaft be additionally connected to a sixth gearshifting element, and the first free shaft additionally to a seventh gearshifting element.
Several other multi-speed transmissions are, for example, also known from the Applicants DE 101 15 995 A1, where four shiftable, planetary gearsets connected to one another and six or seven frictionally engaged gearshifting elements are provided, by way of whose selective engaging a rotational speed of an input shaft of the transmission can be transferred to the output shaft of the transmission, such that nine or eleven forward gears and at least one reverse gear can be selected. Depending on the shift pattern, two or three gearshifting elements are engaged in each gear, so that when shifting from one gear to the correspondingly next higher or next lower gear, in order to avoid range shifts, only one engaged gearshifting element is disengaged at a time and one previously disengaged gearshifting element is shifted.
Further, in the Applicant's generic, unpublished patent application DE 10 2005 002 337.1, a multi-speed transmission with an input shaft, an output shaft, four individual planetary gearsets connected to one another and five gearshifting elements are proposed in which eight forward gears can be selected without range shifts, such that when shifting from one forward gear to the next higher or next lower forward gear, only one of the previously engaged gearshifting elements is disengaged and only one of the previously disengaged gearshifting elements is engaged. The multi-speed transmission also features one reverse gear. In all forward gears and in the reverse gear, three gearshifting elements are engaged at a time. Regarding the kinematic coupling of the four planetary gearsets to one another and to the input and output shafts, it is provided that a carrier of the fourth planetary gearset and the input shaft are connected to one another and form a first shaft of the transmission; a carrier of the third planetary gearset and the output shaft are connected to one another and form a second shaft of the transmission; a sun gear of the first planetary gearset and a sun gear of the fourth planetary gearset are connected to one another and form a third shaft of the transmission; a ring gear of the first planetary gearset forms a fourth shaft of the transmission; a ring gear of the second planetary gearset and a sun gear of the third planetary gearset are connected to one another and form a fifth shaft of the transmission; a carrier of the first planetary gearset and a ring gear of the third planetary gearset are connected to one another and form a sixth shaft of the transmission; a sun gear of the second planetary gearset and a ring gear of the fourth planetary gearset are connected to one another and form a seventh shaft of the transmission, and a carrier of the second planetary gearset forms an eighth shaft of the transmission. With respect to the kinematic coupling of the five gearshifting elements to the four planetary gearsets and to the input and output shaft, it is provided that the first shifting element is arranged in the force flow path; between the third shaft and a transmission housing; the second gearshifting element, between the fourth shaft and the transmission housing; the third gearshifting element, between the first and fifth shaft; the fourth gearshifting element, either between the eighth and second shaft or between the eighth and sixth shaft, as well as the fifth gearshifting element, either between the seventh and fifth shaft or between the sixth and eighth or between the fifth and eighth shaft.
Automatic transmissions in planetary design for motor vehicles have already been described several times, in general, in the prior art, and are subject to permanent further developments and improvements. These transmissions should thus have a sufficient number of forward gears, as well as one reverse gear and a gear ratio that is highly suitable for motor vehicles with a high overall transmission-ratio spread, as well as favorable step jumps. They should further make possible a high starting ratio in the forward direction and have one direct gear, as well as be suitable for use in both private and commercial vehicles. Besides, these transmissions should be simple to construct, in particular requiring a small number of gearshifting elements and avoid double shifts on sequential shifting so that, when shifting within defined gear ranges, only one gearshifting element is shifted each time.
The present invention is based on the task of proposing a multi-speed transmission of the type mentioned above with at least eight forward gears that can be selected without range shifts and at least one reverse gear in which the smallest possible number of gearshifting elements are required while using a total of four planetary gearsets. In addition, the transmission should feature a large spread with comparatively coordinated gearshifting and a favorable degree of efficiency, i.e., relatively low drag and gear losses in the main driving gears at least.
This task is solved, according to the present invention, by a multi-speed transmission.